This invention relates to an optical data recording and reproducing apparatus for optically recording and reproducing data on and from a recording medium such as an optical disk.
One form of a prior art optical data recording and reproducing apparatus will be described with reference to FIGS. 1 and 2 before describing the present invention in detail so that the present invention can be clearly understood. FIG. 1 is a block circuit diagram showing the structure of a semiconductor laser driver circuit incorporated in the prior art optical data recording and reproducing apparatus, and FIG. 2 is a timing chart illustrating the operation of the semiconductor laser driver circuit shown in FIG. 1.
Referring to FIG. 1, the semiconductor laser driver circuit incorporated in the prior art optical data recording and reproducing apparatus includes an input terminal 1 for a sample/hold (S/H) switching signal having a waveform as shown in FIG. 2, an input terminal 2 for an information signal b having a waveform as shown in FIG. 2, a semiconductor laser 3 generating an optical output having a waveform as shown in FIG. 2, a hold circuit 4, a comparator 5, a buffer circuit 6 generating an output signal d having a waveform as shown in FIG. 2, analog switches 7 to 10, a photo detector 11 built in a semiconductor laser package, a reference voltage generator circuit 12, and an input terminal 13 for a write/read switching signal c having a waveform as shown in FIG. 2.
The operation of the semiconductor laser driver circuit in the prior art optical data recording and reproducing apparatus having the structure shown in FIG. 1 will now be described.
In a data read mode, the sample/hold switching signal a is in its low level, and the analog switches 7 and 8 are turned on, while the analog switch 9 is turned off. The analog switch 10 supplies a read reference voltage from the reference voltage generator circuit 12 to one of the input terminals of the comparator 5. At this time, the laser beam source 3 generates its DC optical output. A current proportional to the DC optical output of the laser beam source 3 flows out from the photo detector 11 and is converted into a voltage which is supplied to the other input terminal of the comparator 5. The comparator 5 acts to apply a power servo action to the laser beam source 3, so that the optical output of the laser beam source 3 can be maintained constant.
In a data write mode, a write reference voltage from the reference voltage generator circuit 12 is supplied through the analog switch 10 to the comparator 5, so that the optical output of the laser beam source 3 attains its write power level. Thereafter, the sample/hold switching signal is turned into its high level, and the analog switches 7 and 8 are turned off so as to hold the current corresponding to the write power level of the optical output of the laser beam source 3. Further, the optical output of the laser beam source 3 is modulated by the information signal b. As described above, the power servo action is kept applied until the optical output of the laser beam source 3 attains its write power level, and then the optical output held in that level is modulated by the information signal b.
However, in the case of the prior art structure shown in FIG. 1, the laser beam source 3 generates the optical output of write power level over a long period of time of about 5 to 10 .mu.s in the data write mode, and, depending on the write power level, the optical output of the laser beam source 3 tends to exceed its allowable value (CW). Thus, the prior art structure has had a problem that writing the information with such a DC optical output on a recording medium such as an optical disk leads to undesirable deterioration of the recording medium or undesirable deterioration of the laser beam source 3.
Further, the prior art structure has had another problem that, due to the source-drain discharge of the analog switches 7 to 9 at the time of switching from the data write servo mode to the data write hold mode and also due to the gate-source capacity and gate-drain capacity of the analog switches, the level changing edge of the gate signal is superposed on the source-drain voltage of the analog switches 7 to 9, and the current supplied to the laser beam source 3 increases to cause generation of an excessively high optical output from the laser beam source 3, with the result that the laser beam source 3 will be deteriorated or unnecessary pits will be written on the recording medium.